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What I did to mod the voltage on my 120mm fan on my case was I cut the plug so it fitted onto the molex the other way round so it's now running off 5v. works just as well as whats described above but its a helluvalot simpler

"Immortality notwithstanding, I'm not going to live forever, you know." - o

question: i'm using a 120mm fan that at 12v is rated at 69 cfm and 36dba. I've tested it at 7volts and it is of course noticeably slower and quieter. is there some formula to let me know what cfm i could expect at this voltage (and dba)?

How is it that you can have one wire going to +12v and one wire to +5V and get 7v? Why do these take away instead of adding up?
And how is it that when you switch the black ground wire away from the ground and into the 5v that this works at all now that the ground is no longer there?
It would seem to me that the two voltages would buck and burn things out if this was to work at all. This is DC so I would think that since everything is going one way that it wouldnt work.

How is it that you can have one wire going to +12v and one wire to +5V and get 7v? Why do these take away instead of adding up?
And how is it that when you switch the black ground wire away from the ground and into the 5v that this works at all now that the ground is no longer there?
It would seem to me that the two voltages would buck and burn things out if this was to work at all. This is DC so I would think that since everything is going one way that it wouldnt work.

What gives?

It's not going one way. The 12v is pushing in one "direction" if you will, and the 5v is pushing in the opposite direction. So the first 5v of the 12v line are canceled out leaving only 7v remaining. Now that is in no way what I would consider an accurate technical explanation, but it does make the point in an easy to understand manner.

But where is the ground? I don't understand how this completes the circuit without a ground.
I am wondering how it is possible to use a wire to output voltage and current and yet still perform the job of a return.

But where is the ground? I don't understand how this completes the circuit without a ground.
I am wondering how it is possible to use a wire to output voltage and current and yet still perform the job of a return.

Very interesting stuff!

The 5v wire is the "ground" with the left over 7v running through it to complete the circuit.

I used the 3.3v wire as a "ground" to mod my Panaflo to run at 8.7v (12v-3.3v=8.7v).

When you think of voltage, you must think in terms of difference in potential energy. Voltage is a difference between two points. A simple example could be a ball at the top of a hill. The higher up on the hill, the bigger its potential energy. If you took 4 3v LEDs and wired them in series powered by a 12v source. The measured voltage between each led would be 3v.

Saying that there is difference in potential makes sense in a way to me. There are two wires to complete a circuit.

But the part I can not get around is that this method does not appear to include a positive and negative side.

I can almost imagine this working with a/c because I know that current can travel in two directions with a/c. But in a/c I don't believe this would be the case as if one were to do this with a/c, I believe the two voltages would buck.

But in d/c don't we need the difference to be between a positive and negative, the negative supplying the electrons(current) to the positive side?
I am imagining a circuit that is being created with this mod having two positive sides and no negative.

Ooooohhhh, wait a minute.. You are saying that because one side is lower in voltage than the other that it will give up electrons to a higher voltage source? So, a positive with only 5v will now act as a negative and give up electrons to the side with 12v? The 12v has a stronger attraction and wins over?

this is an easy way to do it yes, but if somone has a bit of knowledge in electronics you can easily set up a NE555 timer as a PWM modulator with a transistor and controll your fan speeds very acurately. If anyone is interested let me know and ill post the information. PWM (pulse width modulation) is very acurate and can run a fans speed slower than a voltage modification, because the PWM controller runs the fan at its desired voltage the fan will start at very low speeds.